1. Field of the Invention
The present invention relates to a receiver for use at a mobile terminal, and more particularly to a receiver that is capable of receiving a plurality of communications complying with different radio communication specifications.
2. Description of Related Art
One example of a conventional receiver is disclosed by EP1098449 and will now be described with reference to FIG. 11, which illustrates a typical example of a conventional receiver. The receiver shown in the figure is a direct conversion receiver that is capable of receiving communications complying with different radio communication specifications. The reference numerals 100 and 101 indicate band-pass filters (hereinafter referred to as the BPFs), which allow signals within a desired frequency band to pass and attenuate spurious signals. The reference numerals 102 and 103 indicate low-noise amplifiers (hereinafter referred to as the LNAs). Components 100 and 102 are used to receive signals within frequency band 1. Components 101 and 103 are used to receive signals within frequency band 2. The reference numeral 104 indicates a selector switch for connecting the output of component 102 or 103 to the inputs of frequency conversion mixers 105, 106 (hereinafter referred to as the MIXes) at the next stage in accordance with the operating frequency band. This selector switch is electronically implemented by a transistor or diode. The input signals to components 105 and 106 are mixed respectively with local signals Lo and LoB, which have the same frequency as the input signals, in order to output a baseband signal. Local signals Lo and LoB have the same frequency but are 90 degrees out of phase with each other. These local signals are generated by a 90-degree phase shifter 109 and a local signal generator 110. Output signals 105 and 106 pass through low-pass filters (hereinafter referred to as the LPFs) 107, 108 respectively for spurious signal attenuation purposes. As a result, baseband I/Q signals are generated. The LPFs 107, 108 are capable of varying their characteristics so as to comply with the radio communication specification for the currently received communication.
In an embodiment disclosed by EP1098449, the number of LNAs must be equal to the number of operating frequency bands because the selector switch is positioned between the LNAs and MIXes. Therefore, the area of the receiver is increased. Further, the receiver's sensitivity is degraded due to an increase in the number of operating frequency bands. These problems will now be described with reference to FIGS. 12 and 13.
FIG. 12 is prepared by the inventor of EP1098449 with reference to FIG. 11 to describe the configuration in which the number of operating frequency bands is increased. It is presumed that the circuitry enclosed by broken line 114 is integrated into a single IC. The LNAs are generally positioned close to the IC's LNA pin in order to avoid performance degradation due to parasitic components. It is therefore difficult to arrange a plurality of LNAs so as to encircle a MIX. Consequently, an increase in the number of operating frequency bands causes an increase in the LNA-to-MIX interconnection distance. When the direct conversion method is employed, no image signal exists in principle. It is therefore common that the LNAs and MIXes are directly interconnected without inserting an image attenuation filter in between. As a result, the signal level lowers due to the interconnection's parasitic capacitance 113, thereby degrading the receiver's sensitivity.
FIG. 13 is prepared by the inventor of EP1098449 to describe the configuration for solving the problem of sensitivity degradation encountered in the configuration shown in FIG. 12. Since sensitivity degradation is caused by the parasitic capacitance of the LNA-to-MIX interconnection, it can be avoided by using MIXes 115 and 116 for frequency band 3, optimizing the layout of LNA 112, MIX 115, and MIX 116, and minimizing the length of interconnection among LNA 112, MIX 115, and MIX 116. However, this solution uses a larger number of MIXes than in the configuration shown in FIG. 10, thereby causing an additional increase in the area of the receiver.